"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."

- Jonathan Swift

December 4, 2012

Encarsia inaron

On this blog, we have covered many stories of either parasite cleverly evading the host's defences or the host valiantly fighting back against these bodily invaders. But sometimes, both parties lose out on this fight, and today we are looking at such a case.

Encarsia inaron is a tiny parasitic wasp no longer than 0.5 mm in length. It was introduced into North America in 1989 from Europe to control the ash whitefly (Siphoninus phillyreae) a sap-sucking insect which itself hails from Europe and the Mediterranean, and has become an established pest in North America and elsewhere in the world. Encarsia inaron lays its eggs in the nymphal (immature) stages of whiteflies. Like most parasitoids, the wasp larvae use the host's body as an incubator and a larder until they are ready to mature into adults, at which point they kill the host by bursting out of its body.

In addition to the ash whitefly, which it was introduced to control, E. inaron also infects a number of other whiteflies (as you will see below). For long-time readers of this blog, you might remember earlier in the year we featured a parasitic wasp that infects aphids and why picking the right-sized host is very important for the survival of its offspring. This also applies to E. inaron but in a different way. If the wasp infects a whitefly nymph that is too far along in its development, then the host would reach adulthood before the wasp larva can complete its development. And unlike other parasitoid wasps, E. incaron is incapable of delaying its host's developmental schedule.

Once the whitefly becomes an adult, rarely will the wasp ever emerge as an adult. While it may seem that in this case the whitefly has won simply by reaching maturity before its parasitoid, that is not exactly the case. Instead, it is a pyrrhic victory - the adult whitefly is still carrying the wasp larva inside it and this burden reduces the number of eggs that it can produce by more than half and significantly shortens its lifespan.

Considering the cost of infecting older nymphs (potentially never reaching reproductive maturity), you'd think this would provide an incentive (or to be more technically precise, evolutionary selection pressure) for E. inaron to avoid older whitefly nymphs - but that was not what the researchers found in the study we are featuring today. When they exposed female E. inaron wasps to two different whitefly species - the silverleaf whitefly (Bemisia tabaci) and the banded-winged whitefly (Trialeurodes abutiloneus), they displayed no particular preference for younger or older nymphs.

So why has E. inaron not evolved the ability to distinguish hosts of different ages? After all, other species of parasitoid wasp, such as the aphid parasitoid mentioned above, have evolved the ability to distinguish hosts of different size and shows a preference for hosts of a particular size.

Keep in mind that this tiny wasp is a generalist that infects multiple species of whiteflies - different species of whiteflies might impose different selection pressures upon the wasp population that prevents them from evolving an optimal approach to selecting the right host. In addition, older whiteflies are likely to be already parasitised by another wasp larva. If a newly arriving larva finds itself in an already occupied host, it can speed up its own development by exploiting the gains of the older, resident larva (a weakened host with an already suppressed immune system). A previous study has shown that when it comes to within-host competition, for E. inaronlate-comers often wins.

So instead of being maladaptive, E. inaron that infect older whitefly nymphs may in fact be taking a bit of a gamble - a highly risky one, but one that comes with a potentially high pay-off.

Reference:
Brady, C.M. and White, J.A. (2012) Everyone's a loser: parasitism of late instar whiteflies by Encarsia inaron has negative consequences for both parasitoid and host. Annals of the Entomological Society of America105:840-845.

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